Software defined networks (SDNs) decouple the data and control planes. The data plane, which is also referred to as forwarding plane or user plane, is the part of the network that carries data packets (i.e., user packets) traffic. In contrast, the control plane in a network controls signaling traffic and routing.
In a forwarding element (e.g., a hardware switch or a hardware router), the data plane is the part of the architecture that decides what to do with the packets that arrive at the ingress interface. The data plane of a forwarding element is implemented by hardware and firmware while the control plane is implemented in software to provide for a more flexible management of network components from a central location.
Performing operations by the software in the control plane could, however, be time consuming and slow. For instance, marking failed ports, marking failed paths, or performing bidirectional forwarding detection for monitoring links between two forwarding elements can place a heavy burden on the processor and slow down the operations of the forwarding elements.